Null measuring bridge



June 28, 1949. V R. A. HAMPSHIRE NULL MEASURING BRIDGE Filed Sept. 14, 1944 F llllllllllill INVENTORQ ROBERT A.HAMPSHIRE ATT R EY Patented June 28, 1949 a i Robert'A. Hampshire; West Caldwell,-N. L, slgno'r to Federal Telephone an r tion, New "York,'N. Y;, acorporationotDelaware "m w 1 44441 4 waf s m 5544045 .1.

This 7 invention relates my; null 'rneasurin'g bridge, that is to say, a'means ,particularlysuita ble for determining the nodal point-or'points-in a conductor suitably energized, to develop stand essential that a null-detector be'extremely 's'ens'itive and thisnecessi'tates the use 'ofdelicate elements used in detecting, amplifying, and in+ dicating or measuring currents induced'inthe" null-detector circuits ,byf'i, the energized cable under examination. "i Because of the delicacy and sensitivity of the elements employed in thein'ullf detector, they. are "easily .Qirijiired' or] even de} stroyed by accidental excess voltages which may be developed in the null-detector system. I t i's necessary, therefore, to provide means for protectiri'g the sensitive elements against-theaction of such excess voltages, and to do thiswitli'out ,at

the same time appreciably decreasing the sensi.-,

tivityoof the null-detector.

rrne principalbbjectlof'th present invehtidri is to provide a null-detectorYwhi'ch' will be prOF tected against injury. by such accidentalf excess voltages 'as may be encountered ingen'er'a'l j se of the null-detector and. which willfjretain its necessary extreme sensitiveness 'in determining with ahigh approach toabsoluteaecuracy the null points or-nodes in ahigh frequency cable energized to develop standing waves.

A further object ofthefinvention is to provide a relatively simple, cornpact and durable null measuring bridge which can be adjusted to and maintained in operative condition with minimum effort.- 7 T v With these principal objects irryiewv and some others which will be apparent to those skilled in the art from thedetailed'descriptionhereinafter given of one embodiment offtheinvention, the invention comprises-a nulldete ctor.whose circuit systemis an adaptation of theT-well-known Wheatstone bridge-circuit system, with the in} dicating or metering instrumentlinlthe normally neutral or bridging branch of, the bridge-circuit system, in combination with a probe device, in? cluding a tuned probe-circuit system, arranged unbalance the system and operatethe indicator or meter. This action indicatesthat the-probe to be energized by the cableor the "like under is not-at anodal point of ei-standing wave in the energized conductor under-test, but by shifting the probe along the '-'conductor a pointwiil be found where no transmission-of energy from the conductor to the null-detector'wiil occur "and the" bridge-circuit systemfwill remain in-balanee; theindicator 'or "meter" showing-la milli;reading,-

vilrhe'reby the" location "of the probe device o'n the conductor under-test will determine the nodal point of the standing wave in'it; I 1 1 4:;

"While a null dete'ctor may be'madeadju'stabie;

so as to lie-employed ataplurality or frequencies;

in *nbrmar operationiithehlgh freq n y "f r whicha conductor or cable istobe used may-be predetermined I and a '-null deteetor operative onlyf at "that-predetermined frequency may be employedwitl'i a consequentsimplification-of the null-detector circuit system'-.f T

"fAn; apparatus embodying theinvention in 4 its simplified form will notv' be described in' con eq nyi i h he a j iQm e f Wi in whichz Fig. '2 'is a diagrammatic detailside view; partly in section, illustratingtheflocation of" the probe"- circuit ininduct ye, !eiation' to the circhitsyste'm of the null-detector, and showing the 'electrostaticshield andit'sf'ground;

*Figt-S is n' explanatory diagramof 'a'WheatE stone bridge-circuit system hereinafter referred to inexplaining' the invention. 1 "Referringyfirst; to Fig.' 3, it'will'be seen that it merely indicates the conductive branches o a well known Wheatston'e bridgb cireuit system; certain junctions jof the'se branches being marked with the letters A; B, C; and D'respecti'vely'.

the ratioof h the resistances of the arms A-B,

' I'n thedescription hereiniter-of the apparatus embodying the apparatus corresponding in'a-general way to the points A, B, C, and-l) of the diagram-Fig.3 will be 'marl-:e;1 :v i Theequivalent current paths of the apparatus corresponding to the armsL-iA-FB,

hrideew j t s m invention, the points in the B-C, A-D and D-C, may be referred to generally as the conductive ar the current path of the apparatus corresponding to the bridge-conductor B--D will be designated the bridgebranch, and the current path in the apparatus corresponding to the conductor extending from A directly to C in Fig. 3 will be designated generally the return path C-A.

In the apparatus embodying the invention the indicating device is most advantageously a very sensitive direct current meter, which as before stated is included in the bridge-branch BD with some accompanying protective elements or instrumentalities, as hereinafter pointed out. Also, suitable detecting and amplifying devices cooperatively associated with each other and with their proper circuit connections are inserted in the arm A-B. The resistances of the remaining arms, B-C, AD and -1), are made such that, under normal conditions, there is no current through the bridge-branch 3-D and the indicating device or meter shows a null indication.

In addition, the probe-circuit is arranged to have a portion in inductive relation to the grid circuit of the detector. If the probe circuit is energized from a conductor or cable, it correspondingly energizes the detector which in turn afiects the amplifier and results in a change of the electron fiow in the amplifier plate circuit. This is equivalent to a variation in the resistance of the arm A-B, thereby causing an unbalancing of the bridge-circuit system, with a flow of current over the bridge-branch B-D of the system and an actuation of the indicator.

With this general explanation of the invention, the details of a practical embodiment of the invention, illustrated diagrammatically in Fig. 1, will now be described.

In this drawing, V1 is a vacuum tube detector having a plate III, a grid II and a filament I2, these elements constituting what is termed a triode. The filament is included in a battery circuit energized by a battery BAi, the circuit including a high frequency choke coil 12 and being bridged by a fixed condenser Ca. A suitable switch S1 is provided for opening and closing the battery connection to the circuit.

The positive pole of the battery is grounded at 21, on a metallic base plate of the apparatus, whose outline is indicated by the dash lines X, Fig. 1.

At V2 is indicated a vacuum tube amplifier, in this case a pentode, having in effect, three grids,

that is to say, a normal or control grid II, a-

screen grid I3, and a suppressor grid I5, respectively. Also the pentode has the usual cathode, in this case shown as a filament I6, and has a plate indicated at I1.

The filament I6 is included in a battery circuit energized by a battery BAa, the negative side of this battery being in conductive connection with the suppressor grid I over a conductor I8.

The amplifier plate I1 is connected by a conductor I 9, to the bridge-branch of the bridgecircuit system at the point B. The screen grid I3 is connected to the conductor I9 by a branchconductor 20.

The detector plate III isin conductive connection with the positive pole of amplifier battery BA: over a conductive path, comprising conductor 2|, a high frequency choke coil L3, conductors 22 and 23, resistance R1 and conductor 24. The amplifier control grid I4 is conductively connected to conductor 24 through a resistance R2.

, rectifier, as indicated at CR2,

The choke coil In is connected at each end to a condenser. these condensers C4 and C5 being grounded on the base plate X of the apparatus.

The closed circuit which includes the inductance In, the two condensers C4 and C5 and their ground connections constitutes a loop circuit which may be constructed or adjusted to have a natural frequency equal to or a multiple of that of the high frequency normally employed in developing standing waves in the cable or conductor under examination, thereby forming a barrier to the passage of any alternating currents, and particularly to any having the frequency which is natural to the loop.

It will be noted that the detector plate III is in conductive connection with the suppressor grid I5 of the amplifier, over the conductor 2|, the inductance In, the conductors 22 and 23, the resistor R1, the conductor 24, the amplifier battery BAa, through a part of the said battery circuit, to and over the branch I8. Also the detector plate In is in conductive connection with the control grid I4 through part of this circuit and the resistor R2.

The negative pole of the detector filament battery BAi is equivalent to the point A in the diagram Fig. 3. It is connected by a conductor 25 to one end of a fixed resistance Ru whose other end is connected to one end of what may be called a rheostatic resistance R5, along which may beadiusted one end of the bridge-branch in which is included the indicating device, which, most advantageously is a direct current meter, indicated at M1. The neutral position of the ad- Justable end of this bridge-branch may be considered the equivalent of the point D in the diagram Fig. 3. The other end of the bridge-branch is the point B to which the amplifier plate I! is connected by the conductor I9, heretofore referred to.

In the present example there is provided a protective branch, shunt or by-pass around the meter M1 which includes a resistor having a negative coefiicient of resistance, such as a selenium and in this case a resistor R118 put in series with the meter to somewhat increase the impedance of the meter and thereby maintain a proper normal potential drop over the selenium rectifier. At normal voltages, the selenium rectifier has a very high resistance, but with high voltages the resistance of the selenium rectifier falls rapidly and thereby protects the meter by allowing any abnormal current to flow over the by-pass or shunt branch in which the selenium rectifier is placed.

A resistor Re is connected at one end to the bridge-branch, at point B, and at the other end to an end of the resistor R5. The Junction point of the connection of these two resistors is equivalent to the point C on the diagram, Fig. 3, and is so marked in Fig. 1. Hence the branch containing the resistor R3 is the arm 3-0 of the bridge circuit system, and the branch containing that part of the rheostatic resistance to the left, Fig. 1 of the adjusted null position of point D is the equivalent of th arm DC of the bridgecircult system, Fig. 3.

The point C is grounded on the .base plate of the apparatus, as indicated at 26, and the path from point C to point A at the negative pole of the detector filament battery BAi, which includes the ground connection 28, the base plate X, the ground connection 21, and the battery BA; may be called the return path equivalent t0 the Path AC in Fig. 3.

' ductive primary.

very high so that the potential on-grid I W w v tively that of conductor I 4, i.e.,that of the positive end of resistor R1 .When the current through R1 The equivalent oi the arm A-b in Fig. 8 is pole oi-the detector over the conductor 25, and includes the fixed resistor Re and so much of the resistor Rs as lies to"theright1oi:the adjustable point D,-Fig. 1, whenthe bridge-circult system is in balance under normal. conditions.

An important feature of the-invention'is that the grid ll of the detector triode forms the terminal of a conductive pathleading to its plate 'circuit,-ior example at the junction point conductors '22 and 23, this path in'cludlnga conductor 30, a curvedportionjl, whichiorpurposes ofidentiflcation may becalled an inductive secondary and which is electrostatically shielded by a split metallic shieldxfl; grounded at 335a conductor 34, a highirequency choke coilLi and a conductor 35. Y The conductor .34--

the path from the negative filament battery BAi,

is connected to ground at 38; through a fixed condenser Ca, and the 'con- 3 w ductor 30 is connected to ground at 31 through a condenser C2 which in th ebest embodiment of J the invention is a variable .condenser,-serving to tune the circuit, which'includes the conductors 3ll, 3|,-34,uthe and the ground connectiomto resonate cat'the high frequencyof the energy in the cable under For the purpose of energizing the gridbranch containing the curved portion 3|, which serves as an inductive secondary, a: probeecircuit, is provided, this comprising a curved portion" which is'intended to be brought into magnetically inductive relation with thecurved portion 3| of the grid branch and The probe-circuit also-comprises a pair of contact devices 4|,a 4la. connected by conductors 42, 43, to the respective ends of the curved portion '40, these, conductors being bridged by a variable parallel with the curved portion 40 and serving to tune the probe-circuit to synchronism with the high frequency energy in the cable under test. The said curved portion 40 .oithe probe-circuit may be brought into magnetically inductive relation with the curved portion 3| of the grid branch in any suitablevway, asiorexample by placing it above thecurved portion 3|, as indicated in'Fig. 2, and may be adjusted lengthwise to varythe extent of coupling of the twocurved portions, and then maybe fixed in the adjusted relationship by any suitable means (not shown). In general, the coupling between the curved portions 40 and 3| is advantageously kept loose, to reduce the eifects of mutual induction ,Whileit is satisfactory inc-some uses of connection, in th best embodiment oi the invention it is hig'hlyadvantageous to employa .s'e-,,,

lenium rectifier CR1, i i-combination with, the resistors R1 and R2, the selenium rectifier being included in a branch conductor, illv extending from conductor 23, a in parallel :with resistors R1 and R2 to grid,

- When the current through R1 age on the rectifier CR1 is ,lowand its resistance is 14 isefiecincreases the resistance of CR1 correspondingly decreases so that the potential on, the control grid |4 becomes increase of current through Rnwhich of. course,

more negative. with sufflcient two condensers C2 and Cato thereby serve asan incondenser C1 arranged in the invention'to omit the resistor Reandconnectthe v control grid to the conductor 23,; by a simple is small, :theyoltis due'to a corresponding increase in current through ivacuum tube V1 thev decrease.- in resistance of the selenium r rectifier; CR1 constitutes an effective shortcircuit betweenthe grid VH and conductor 3 2-3, and thus. the. voltage: von control grid l4 becomesmore negative, 138. effectively it becomesthat of the negative endof resistorvRi.

as compared with'whatit wouldbe without the examined at high frequency is connected and energizedto impress upon one end of it the predeterminedhigh frequency voltage so as to develop in the cable a seriesof standing waves.

The contacts 4| and-4|aare=brought in contact with the cable, and can be adjustedback and forth' lengthwise 'oi thecable, 'atest -b'eing made at each adjusted position. As'suming that atthefirs't:positionof the adjustment the contacts 4| and 4la are not at a nodal point'of the standing waves in the cable; a high frequency current: will 'flow in the probe-circuit, which :may be tune'd to resonance 'by adjusting'the variable condenser vC1. The'curved portion 40 of the probe-circuit, being. in inductive relation to the curved; portion-3| of'the detector grid branch, inducesc'a high frequency voltage in th'e giid branch,.and thereby impresses ayoltage on the detector grid II, the maximum amplitude of'this voltage being controlled by the adjustment of the variable condenser C2; The signal thereby received on the detector grid ll, affects the flow of electrons from the'detector filament to its plate, in thexwell-lmown nianneigso as to correspondinglyalter'the plate current-from the detector and this effect is the equivalent of a change offlresistancei-of the arm A---B of the bridge-circuitrsystem'which results -in unbalancing the bridge-ecircuit system" so that a "current-flows across the. bridge branch 3-D and actuates'the meter. 4 This indicates *that l the probe-contacts are not at the nodalpoint: of a'standing wave. Thereupon they are moved to another position 5 and another test .made until a position is reached :whereqtheindicatorxor meter-is-not actuated but .remains in its zero; or 'null position. :"Ihis indicates thatthe probe=contacts 4|, Maare properly positioned at a nodal point of a1 standingwave meter is used as the indicating means, the value of current shown by the meter aids in determining the ,eIIOr in the position of the probe, and the extent to which-'itjj must be shifted to reach 60, the nodal position, 5; j. a p It'isto be' noted that thedetector grid takes its biasirom the plate circuit of' the ..detectqr, over the conductor 35, inductance L conductors 34, 3|and -3'0 ,\this grid'bias path beingconductive for direct current but blocked against emernating currents by the high frequency choke coil L and the loop, circuitff for'med by the. high'irequency choke coilLa', condensersci'andCs and their ground connections, IHence, anychange in 7o thevoltage in the cable or conductorqbeing tested. Where a n nature, aids in protecting the apparatus against unusually strong signals, such as might result if the probe-contacts 4| and lie made contact with the cable under test at a point far removed from a nodal point of a standing wave in such cable.

Also, the control grid ll of the amplifier tube V: is biased from the detector plate circuit because of the connection of CR1 and R2 across the resistor R1 so that an increase in plate current of the detector tube V1 causes a corresponding decrease in voltage on the amplifier tube control grid H with a consequent decrease in the fiow of electrons from the filament to the plate of the amplifier tube.

The action of the circuit is, therefore, such as to provide an extremely sensitive indication of signal changes when the signal is small, and a very insensitive indication when the signal is large, this latter insensitive response being desirable since it affords overload protection to the elements M1 and V1. When the signal is small a slight change will result in a substantial increase of plate current in V1. The change will not be suflicient, however, to materially decrease the electrical resistance of rcctifiers CR1 and CR2. Meter M1 is shunted, therefore, by a resistance (e. g. Re) which is high and the meter indicates the full effect of the slight signal change. Amplifier tube V: is furthermore protected by CR1 from a large change in control grid voltage and its resistance to the fiow of current from plate IT to filament I6 is low; its eflect as an impedance to flow of detector plate current is, therefore, correspondingly low.

When the signal is large, as a result, for instance. of a displacement of the probes far from a nodal point, the detector current tends to rise to an extent which is dangerous to meter M1 and to the detector tube Vi. M1 is protected from this action by CR2, and V1 is protected by the action of the amplifier tube V2. This latter action results from the increase in resistance between plate I! and filament it of V: which is brought about by the increased negative control grid voltage. The action is further enhanced by the connection of grid H of tube V1 which is such as to provide a large negative bias to this tube which further protects it.

It has been found that the apparatus constructed as described is extremely sensitive and successful in locating the nodal points with a close approach to absolute accuracy, while at the same time it is well protected against excess voltages such as are occasionally encountered in actual practice.

What is claimed is:

1. In measuring apparatus of the class described, a Wheatstone bridge circuit which has four arms and two diagonals and which is adapted to be energized by an electric power source connected across one diagonal thereof to actuate indicating means connected across the other diagonal thereof to indicate the condition of balance of said bridge circuit; one of the arms of said bridge circuit including first and second vacuum tubes, each having a cathode, an anode, and a control electrode; circuit means connecting said tubes in series between the terminals forming the ends of said arm of said bridge circuit with the anode of said first tube connected to the cathode of said second tube; biasing means interposed in said last-named connection; means connecting the anode of said first tube to the control electrode of said second tube; means for impressing the signal to be measured between the oathode and control electrode of said first tube; and means providing a degenerative feedback connection between the control electrode and the anode of said first tube, whereby said tubes and indicating means are protected from the detrimental eil'ects of large signals.

2. In measuring apparatus of the class described, a Wheatstone bridge circuit which has four arms and two diagonals and which is adapted to be energized by an electric power source connected across one diagonal thereof to actuate indicating means connected across the other diagonal thereof to indicate the condition of balance of said bridge circuit; one of the arms of said bridge circuit including first and second vacuum tubes, each havinga cathode, anlanode, and a control electrode; circuit means connecting said tubes in series between the terminals forming the ends of said arm of said bridge circuit with the anode of said first tube connected to the cathode of said second tube; a biasing impedance interposed in said last-named connection; degenerative feedback means connecting the anode of said first tube to the control electrode of said second tube; means for impressing the high frequency signal to be measured between the oathode and control electrode of said first tube; and high frequency filtering means conductively connecting the control electrode circuit of said first tube to its anode, whereby said apparatus is protected from the effect of signals of excessive magnitude. a

3. In measuring apparatus of the class described, a Wheatstone bridge circuit which has four arms and two diagonals and which is adapted to be energized by an electric power source connected across one diagonal thereof to actuate indicating means connected across the other diagonal thereof to indicate the condition of balance of said bridge circuit; one of the arms of said bridge circuit including first and second vacuum tubes, each having a filament, an anode, and a control grid; two sources of D. C. power, each connected across the filament of one of said tubes to heat said filaments; circuit means connecting said tubes in series between the terminals forming the ends of said arm of said bridge circuit with the anode of said first tube connected to the positive side of the D. C. filament power source of said second tube; biasing means interposed in said last-named connection; means connecting the anode of said first tube to the control grid of said second tube; means for impressing the signal to be measured between the filament and control grid of said first tube; and means connecting the negative side of the D. C. filament power source of said first tube to one terminal of said bridge arm and the positive side of said D. C. filament power source to the diagonally opposite terminal of said bridge circuit, whereby said bridge circuit is energized by said D. C. filament power source.

4. In measuring apparatus of the class described, a Wheatstone bridge circuit which has four arms and two diagonals and which is adapted to be energized by an electric power source connected across one diagonal thereof to actuate indicating means connected across the other diagonal thereof to indicate the condition of balance of said bridge circuit; one of the arms of said bridge circuit including first and second vacuum tubes, each having a cathode, an anode. and a control electrode; circuit means connecting said tubes in series between the terminals forming the ends of said arm of said bridge circuit 9... withthe anode of said first tube connected to the cathode of said second-tube; biasingmeans interpose'dfin said last-named connection? means I for impressingthe signal tobe measured betweenz= the cathode and control'electrode of said flrstws tube;f-first impedance 'means connected to the: cathode of said second tube; second impedance meanshaving anegative coeflicient ofnimpedance' i connected between said firstimpedancemeans and the anode of said fir'sttube to form a shunt l across'said biasing means; and means connecting the control electrode of saidsecond tube to the junction between said first andsecond imped-- ance s' rneans,'whereby the negative bias'on the 2 control grid of said secondtube is increased as ll anode, and a control grid; circuit'means connects ing said tubes in 'seriesbetween the terminals forming theends-of said arm of said bridge cir-,

cuit with the anode of said first tube connected to the cathode of saidsecond tube; a1biasing resistor interposed in. last-named connection; means for impressing the signalto' be measured between the cathode and control. grid. of said first tube; a resistor connectedto thecathodeof said second tube; a selenium rectifier connected between said resistor and the anode of said first tube thus forming a shunt across said biasing resistor; grid of said second tube and the junction between said resistor and said rectifier, whereby the negative coefiicient of resistance of said rectifier causes the negative bias on the control grid of said second tube to be increased as the magnitude of the impressed signal is increased.

6. In measuring apparatus of the class described, a Wheatstone bridge circuit which has four arms and two diagonals and which is adapted to be energized by source connected across one diagonal thereof to actuate indicating means connected across the other diagonal thereof to indicate the condition of balance of said bridge circuit; one of the arms of said bridge circuit including first and second vacuum tubes, each having a cathode, an anode, and a control electrode; circuit means connecting said tubes in series between the terminals forming the ends of said arm of said bridge circuit with the anode of said first tube connected 50 to the cathode of said second tube; biasing means interposed in said last-named connection; means connecting the anode of said first tube to the control electrode of said second tube; means for impressing the signal to be measured between the cathode and control electrode of said first tube; first impedance means connected in series with said indicating means; and second impedance means having a; negative coefiicient of mpedance connected across the diagonal of said .ridge circuit in parallel with. said indicating means, whereby said indicating means is protected against the detrimental effects of large signals.

'7. In measuring apparatus of the class dediagonals and which is 20 one of the, 25

and a connection between the control in an electric power 50 10 scribed, a Wheatstone bridge circuit which has four: arms rand two diagonals and which is adapted to be energized by actuate indicating meansconnected across the other; diagonalathereoi toj indicate the, condi-- tion of, balancesof said bridge circuit; one. of

thearmsof saidbridge circuit including first 1 and second vacuum tubes eachhaving a cathode.

an anode, and a control electrode circuit means "connecting saidtubes in series. between theter of said arm of said. bridge-circuit-withtheanode of said first tube connected to; the cathode of said second tube; biasing means interposed in, said last-named conf nectionpmeans.connectingthe anode of saidfirst' tube tothecontrol lectrode of said second tube; meansv for impressing .the signal to be measured minals :forming the; ends between the cathode a d control electrode of said first :tube; means. providing a degenerative ree -y. .,;back connection between the .contr o1 electrode.

and the anode of said first tube; first impedancef,

means, connected in series with said indicating means;,and second impedance means havinga.

non-linear" resistance characteristic connected across the diagonal of said bridge circuit in.

indicatingmeans, whereby j parallel withssaid a 7 said tubes ,and, indicating-means are protected from. the detrimental effects of large signals.

8. In, measuring; apparatus other diagonal thereof to indicate'thecondition of balance of said bridge circuit; one of the arms of said bridge circuit including first and second vacuum tubes, each having a cathode, an anode, and acontrol electrode; circuit means connecting said tubes in series between the terminals forming the ends of said arm of said bridge circuit with the anode of said first tube connected to the cathode of said second tube; a. biasing impedance interposed in said last-named connection; means connecting the anode of said first tube to the control electrode of said second tube; means for impressing the. high frequency signal to be measured between the cathode and control electrode of said first tube; high frequency filtering means conductively connecting the control electrode circuit of said first tube to its anode; a current meter connected across one diagonal of said bridge as an indicating means; a resistor connected in series with said meter; and a selenium rectifier connected across said meter and resistor in the diagonal of said bridge circuit, whereby said apparatus is protected from the effect of signals of excessive magnitude.

9. In measuring apparatus of the class described, a Wheatstone bridge circuit which has four arms and two diagonals and which is adapted to be energized by an electric power source connected across one diagonal thereof to actuate indicating means connected across the other diagonal thereof to indicate the condition of balance of said bridge circuit; one of the arms of said bridge circuit including first and second vacuum tubes, each having a filament, an anode, and a control grid; two sources of D. C. power, each connected across the filament of one of said tubes to heat said filaments; circuit means connecting said tubes in series between the terminals forming the ends of said arm of said bridge circuit with the anode of said first tube an electric. power. source connected across one diagonal thereof to I of the class 'de-" scribed, a:,Wheatstone bridge circuit whichhas I 11! connected through a grid biasing resistor to the positive side of the D. C. filament power source of said second tube; means connecting the anode of said first tube to the control grid of said second tube; means for impressing the high frequency signal to be measured between the filament and control grid of said first tube; means connecting the negative side of the filament power source of said first tube to one terminal of said arm and the positive side of the diagonally opposite terminal of said bridge circuit to energize said bridge circuit; high frequency filtering means conduotlvely connecting the control electrode circuit of said first tube to its anode; an indicating'meter connected across said other diagonal of the bridge; a resistor connected in series with said meter; and a selenium rectifier connected across said meter and resistor, whereby the nonlinear resistance characteristic of said rectifier protects the meter against the detrimental efiects of large signals.

10. In measuring apparatus of the class described, wherein the desired measurement is made with a Wheatstone bridge circuit which has impedance means forming three arms thereof and which is adapted to be energized by an elec tric power source connected across one diagonal thereof to actuate indicating means connected across the other diagonal thereof to indicate the condition of balance of said bridge; a fourth arm for said bridge circuit comprising, in combination, first and second vacuum tubes, each having a cathode, an anode, and a control electrode; circuit means connecting said tubes in series between the terminals forming the ends of the 12 fourth arm of said bridge circuit with the anode of said first tube connected to the cathode of said second tube; biasing means interposed in said last-named connection degenerative feedback; means connecting the anode of said first tube to the control electrode of said second tube for limiting the relative output of said first tube; a tunable highfrequency resonance circuit connected to the anode and control electrode of said first tube; and selective high frequency pickup means coupled to said resonance circuit to intercept and impress thereon a high frequency signal to be measured. whereby said apparatus may determine accurately the null point of a high frequency field without being detrimentally aflected by the maximum field.

ROBERT A. HAMPSHIRE.

nhmnnons orran The following references are or record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,475,749 Chubb Nov. 27, 1923 1,836,934 Morecroft Dec. 15, 1931 1,919,215 Gunn July 25, 1933 2,027,195 Parkin et al. Jan. 7, 1936 2,079,485 Bousman May 4, 1937 2,106,713 Bowen Feb. 1, 1938 2,143,219 Wenger Jan. 10, 1939 2,191,271 Carter Feb. 20, 1940 2,242,874 Usselman May 20, 1941 2,297,543 Eberhardt Sept. 29, 1942 2,302,143 Pickles Nov. 17, 1942 

